Steam-Assisted Gravity Drainage (SAGD) is one technique used in enhanced oil recovery to extract bitumen, heavy or extra-heavy crude oil from a sub-surface formation. It usually comprises the drilling of two parallel horizontal wells with one positioned about 4 to 6 meters above the other. The upper well constitutes an injection well configured to inject high pressure steam into the formation to heat the oil and reduce its viscosity. The heated oil then flows more easily to the lower well, under the action of gravity. The lower well constitutes a production well which collects the heated oil and any water resulting from condensation of the injected steam, and transports this to the surface. Commonly, an artificial lift device, such as an electrical submersible pump (ESP), will be employed to help flow the fluids to the surface.
However, traditional SAGD depends on relatively thick and homogeneous reservoirs for economical drainage. A reservoir which is split into two or more layers separated with horizontal (or near horizontal) rock (e.g. shale) barriers is not likely to be economically producible with traditional SAGD since it would require drilling two wells into each reservoir layer, one for each of the injection and production wells.
It is therefore an aim of the present invention to provide a method for improved gravity drainage, which addresses the afore-mentioned problems.